Electron beam concentrating system



Patented Dec. 22, 1942 2,305,334 ELEClRbN BEAM OONCENTRATING SYSTEM Charles V. Litton, Redwood City. Calif, assignor to International Standard Electric Corporation New York, N. Y.. a corporation oi Delaware Application July 13, 1940, Serial No. 345,284

Claims.

This invention relates to systems for concentrating a beam or electrons and more particularly to means for concentrating a beam of electrons in a velocity modulated tube.

There are many systems in which a beam of electrons is used. In such systems it is often desirable that a beam of electrons be concentrated as much as possible. Fo example, in vacuum tube systems operating on the principle of velocity modulation it is desirable that the circuits controlling the velocity modulation and tor extraction or energy irom the beam be arranged so that no electrons collide with elements in the circuit. However, in order to eiiectively control the electron beam it is necessary that the controlling electric fields be effective substantially throughout the cross-sectional area of the beam.

.Insystems previously pr posed this is accomplished by means of a grid electrode arranged extendinginto theareaoithebeam. Thiselec- .trode is generally formed of flatiinetal sheets arranged edgewlse to'the electronrbeam. Even with this precaution it is found-"that considerable energy. is lost by collision of'electrons with the grid and a consequent heatingthereot. I! meansris provided for concentrating the avery small diametenzthen a single opening ,may be provided in adolescent or the circuit to control the beam and no :grid electrode is required.

- 'It 13 a principal object of my invention to provide magnetic means for concentrating a beam of electrons.

It is a further object of my invention to provide a magnetic circuit having amagnetic field distribution to produce the desired beam concentration.

It is a still further obiect of my invention to provide a velocity modulated vacuum tube in which grid electrodes are not required.

According to my invention I accomplish the above objects and other obiects by providing a magnetic circuit having a fleld or high intensity converging in the direction of travel of an electron beam. This field may be formed by means of electromagnets or permanent magnets arranged so that the lines or magnetic force converge around the beam.

In a velocity modulated tube the beam may be concentrated to a very small diameter. It then the beam is passed successively through resonant cavities to produce velocity modulation, and to extract energy from the beam. only small openings need be provided and no grid in the usual meaning of the term is needed to distribute to electric potential thereacross.

A better understanding of my invention may be obtained from the particular description of particular embodiments thereof taken with the accompanying drawing, in which:

Fig. 1 shows an embodiment used to explain he principles or my invention:

Fig. 2 shows a modification of my invention using a preferred distribution oi magnetic fields and Fig. 3 shows a further modification of my invention using permanent magnets to produce the control.

In Fig. i is disclosed an embodiment of a velocity modulation tube incorporating one form 01 magnetic concentration arrangement in accordance with my invention. In this arrangement is provided a cathode II for emitting electrons which are formed in a beam by any suitable aceelerating means, for example, by application of a positive potential to the other portion or the tube structure such as the resonant cavity. The beam of electrons then may pass through t chambers i2 and II. The resonant chamber It serves to modiiy the veloci oi'theelectronsinthebeamandmaybereierred toas'the'modulationcircuit. so modulated-thenmass through-tube which permits theelectr'ons"to" become bimched' due to their changesinvelocitycausedinchamber II. Resonant chamber-"i8 then may extract energy from. the hundred electrons [or application to'any-suitable circuit not shown. Circuit it will hereafter be rei'crredto as the extractor circuit. Theelectrons after passing circuit ii arethencollectedbycoilectorortargetelectrodedl so as to remove them from possible interference in the circuit.

In a velocity modulated tube it is necessary I that some means be provided for applying a potential stress from modulation circuit I! to the electron stream in order to control the velocity modulation. This is generally accomplished by r n in a grid across the area covered by the stream and connected to circuit I! so that the potential of circuit It will be applied more or less uniformly over the stream. It has been found, however, that even though these grids are designed to be open mesh work, quite a numher of electrons strike the grid and cause excessive heating. Such excessive heating serves as a power limitation to which the'tube can operate. Furthermore. in order to achieve high eiliciency it is desirable that the depth or field penetration at the resonant circuit represents a small fraction of p 1 where v is the wavelength in tree space and n represents the velocity oi the electron beam divided by the speed of light. This result may be obtained either by using a high velocity stream. or by reducing the size of the electrode opening or both.

In accordance with my invention I use no grid at all in connection with circuits l2 and it but use small size apertures such as It, ll, in connection with resonant circuit l2 and in connection with circuit II. These apertures are made suiliciently small so as to preserve substantially uniform controloitheelectronsinthisstream and not permit any great depth of penetration.

In order that the small apertures may be used. I provide in accordance with my invention an arrangement for concentrating the electrons from a relatively large source such as cathode ill to produce a beam having an increased density oi electrons.

It is known that electrons traveling at any given velocity will tend to follow magnetic lines of force in a spiral path which path generally follows the lines of force. Accordingly. I utilize this principle for the purpose of producing a concentration of the electrons emitted from cathode ll. Thus. I provide a magnetic circuit indicated at ll. having a pole 2| and a second pole 22 both preferably, but not necessarily, of annular i'orm. Pole It is placed outside cathode ll, while pole II is arranged with an opening or small diameter. Accordin ly, a substantially frusto-conical ileld of magnetic force is set up betweenpolesli andl2.asshownbydottedlines in Fig l. Electrons emitted from cathode III are thus concentrated by the magnetic held into a smallbeam. Inorderthat-theiulleiliciencyoi the system be retained, however, it is desired that aiurtherconcentratlonortheelectronbeambe madetoassurethatthebeamissumcientlysmall to pan through openings II. II. II and II. In accordance with my invention this maybe accom by anadidtlonal magnetic circuit ll, which is provided with pole pieces ll. 82. Pole piecell isanannulusoirelstivelylargediameter,whilepole piece" isanannulus oismaller diameter. Jhe lattermaybearrangedinsidethe actual tube structure itself. This may-.be. done by platingsott iron material with suitableplatimmaterial andweldingorbrasingittothe nonmagnetictubc envelope material meiera'bly copper. Thesmallgapcausedbythe intervention oi .the copper layerin themagnetic circuit may beoisuchsmalldimensionsastobeunimportant withrespecttothelsrgerairgapsbetweenpole pieces 8| and 82.

In order to secure relatively great concentration it is n that the magnetic field produced by magnetic circuits I! and ll be relatively strong. Accordingly. it is suitable to utiuse electro-magnets for this purpose and to this end I provide magnetizing coils 2! and I3 associated with magnetic circuits in and 30, respectively. It is clear that it suitable magnetic concentration can be obtained. permanent ma nets may be provided for the magnetic circuit instead of electro-magnets as shown. Preierably when electro-magnets are used the outer shell is made with openings (not shown) so that the tube and themagnetwindingsmaybeaircooled.

Furthermore, if desired, windings I3 and it may be provided on the outer portion oi the magnetic circuit instead of being wound within the cores as shown in Fig. 1.

The concentrated beam oi electrons ai'ter passing through apertures i8, II, is conducted to an enclosure 38. However, thebeam may be so highly concentrated that it is desirable to spread the beam somewhat before it is permitted to come in contact with the target electrode Ii. This may be accomplished, for example, by provision of a conical shaped electrode 31 upon which is applied a negative potential which will serve to spread the electron beam outwardly. It is clear tliat other means may be used for spreading the electron beam, such as a transverse magnetic circuit, or a circuit having diver i magnetic lines. or a positive ring electrode tending to attract the electrodes outwardly thereto.

In an actual trial of concentrating arrangements in accordance with my invention, with a beam oi electrons accelerated by a 3000 volt potential and by use of a ileld oi several hundred gauss, substantially all of the electrons irom a diameter cathode were concentrated into a beam of 2 mm. in diameter. This concentration was done in a beam traveling a distance of 5" and having an intensity of 200 milliamperes. that is, an average density oi. about 64 milliamperes per square millimeter. It is clear that. while I have illustrated an application oi my concentrating system to a velocity modulated tube arrangement, other applications or this arrangement may be found useful. Itis clear that the principles or my invention with respect to the concentration of electron beams, while constituting a substantially essential combination in so far as the tube operation is concerned. may have other nelds of application independent oi the velocity modulated tube arrangement.

While I have shown in Fig. 1 an embodiment of my invention (or the 91111 of explaining the operation thereof, it is clear that in some instancesacircuitsuchasshown thereinwlllbe insuilicient to produce the desired concentration. This isparticularlytrueoi themagnetlccircuit related to the resonant chambers oi the tube, sincetheairgapinthissystemmaybesogreat that it is diilicult to secure suilicient concentration of magnet fields at the point adjacent openings ll, l1, and II. InFig.2 is shownamodiiication or my invention, wherein separate magnetic circuits are provided about each or the resonant chambers of a velocity modulating tube. Inthisdisclosureisshownasimilariorm orvelocity-medulating tube having a cathode ll, an accelerating electrode ll. resonant chambers II, II. .with associated control openings ll, l1, and II, il, respectively and a target electrode it. About the cathodeend oi the arrangement is provided a first magnetic circuit 28 with pole pieces Ii, 22. Pole piece 22 is shown as arranged in the tube envelope structure and separated from magnetic circuit 20 by means or the thickness 0! the material therein. It is clear, however, that it desired this pole piece 22 may be sealed within the envelope, in the manner indicated in Fig. 1. When a sealing of this soft iron metal with the copper is provided in a tube structure it is desired that the proper materials be corrugated or moditied in some manner so that unequal expansion oi the two metals will not cause a break in the envelope destroying the vacuum.

Associated with resonant chamber I2, I provide a second magnetic circuit 80, pole pieces 8i, II. Thus the air gap across apertures i8, i1 is relatively small and a strong magnetic concentratin field may therefore be produced so as more eiiectively to stop down, narrow, or focus the beam as it passes through these openings.

Thus a third magnetic circuit I0 is provided about resonant circuit It. This arrangement is provided with a pair of pole pieces ll, 42, which provide a narrow air gap across openings II, I! so as to concentrate the beam while passing therethrough.

Individual energizing coils 23, I3, and N. are provided for magnetic circuits 20, 30 and "I, respectively. These energizing coils should be so energized that the polarity or each magnetic field is properly related one to another to produce the desired conical concentration and not to furnish an interference one with another.

In this arrangement I have disclosed a ring electrode 38 insulated from the wall of enclosure 36, by a strip of insulation 39. This electrode 38 is raised to a small potential so as to spread the electron beam prior to its impingement upon target electrode ii.

In Fig. 3 I have illustrated a third embodiment of my invention. This embodiment is similar to Fig. 2 in that it is a system for providing individual magnetic circuits for the cathode beam arrangement and for each of the resonant chambers l2, it. However, instead of using electromagnets, the arrangement shown in Fig. 3, provides permanent magnets for producing the concentrating field. Accordingly, the magnetic circuit 30 for concentrating the electron stream as emitted from a cathode comprises a permanent magnet 303 which may be made in the form of a tubular ring. Soft iron pole pieces 30!, 302 are provided for producing the concentrating field and these pole pieces are connected with the opposite ends of magnets 303 by means of soft iron rings or spiders 305, I08. The magnetic circuit 3|) about resonant chamber l2 comprises a permanent magnet "3 having pole pieces Mil, 402 and spiders 5, 40B interconnecting the permanent magnets and the soft iron pole pieces. In order that the magnetic circuits may be properly operated and will not interfere with one another it is necessary that magnet 308 and 403, be arranged with like poles toward one another. A third magnetic circuit is provided around resonant chamber l3. This comprises a permanent magnet 503 with soft iron pole pieces till, 502 and interconnecting spiders 505, 588. Pole pieces 502 if desired may be extended axially outward from magnet 503 and may serve as a further pole piece for a fourth magnetic circuit herein provided.

A fourth magnetic circuit is provided for the purpose of spreading the electron beam after it has passed through resonant chamber It. This arrangement comprises a permanent magnet 603, associated by means of a spider or disc arrangement 805 with the extended portion of pole piece 602. Another pole piece which may be a soft iron ring 606 is provided at the opposite end 01' permanent magnet 80!. It will be noted that the air gap between 606 and 502 is such as to provide a diverging magnetic field. Accordingly. the transverse component of this field will tend to spread the electron beam so that upon impingement on electrode ii the energy will not be too highly concentrated.

While I have disclosed certain features of my invention independently in Figs. 1, 2, and 3, it should be distinctly understood that this is done for the purpose'of simplifying the showing. It is clear that the various features of each of these arrangements which properly apply may be readily exchanged for systems shown in the other arrangements, or may be used in addition thereto.

For example, if desired any one of the systems for spreading the electron beam or any combination of these systems may be used in a single tube structure. Furthermore, it should be realized that the magnetic structures shown are merely illustrative of the invention and that any modification in shape and arrangement of this structure may be made within the spirit of my invention. What I consider to be my invention and upon which I desire protection is embodied in the accompanying claims.

What I claim is:

1. .A vacuum tube comprising means for producing a stream of electrons, means for modifying said electron stream, means for extracting energy from said modified electron stream, and a magnetic circuit including a relatively short air gap, said air gap extending along said stream for producing substantially converging lines of magnetic force in the direction of travel of the electrons of said stream to concentrate said stream into a small cross-sectional area. said means for modifying said electrons comprising a resonant chamber including a pair of grids having a single opening in the path of the electron stream, and wherein said magnetic circuit is generally coaxially disposed about said electron stream and comprises a first part between said electron producing means and said opening for producing converging lines of force to concentrate said beams before reaching said opening, and a second part disposed after the last of said openings, said second part producing converging lines of force to maintain said beam concentrated during traversal of said grid openings.

2. A vacuum tube according to claim 1, wherein said first part comprises an electro-magnet having oppositely directed pole pieces, between which magnetic flux lines pass, said oppositely directed portions being of diiferent transverse dimensions.

3. A vacumn tube according to claim 1, further comprising a target electrode generally coaxial with said magnetic circuit, said target electrode receiving electrons from said beam after said electrons have passed said means for extracting energy therefrom, and magnetic means for diffusing the electrons of said beam before striking said target.

4. An electronic discharge device comprising means for producing a stream of electrons, means forming a resonant chamber longitudinally spaced from said electron producing means and in the path of said stream, said resonant chamber forming means including a stream-controlling element having a small opening in the direct path of the electron stream, said opening having a cross-sectional area substantially smaller than the initial cross-sectional area of the produced stream, and means for reducing the size of the produced electron stream to a cross-sectional area substantially less than that of said opening.

5. The combination according to claim 4, in which said last means comprises a pair of annular magnetic pole pieces each encompassing a substantially greater cross-sectional area than the opening in said control element, one of said pole pieces being positioned about and adjacent the electron stream producing means, and the other pole piece encompassing a cross-sectional area less than that of the said one pole piece, polltloned about the electron stream intermediate the electron stream producing means and said control element.

OEARLEB V. IJTI'ON. 

